Stereoselective Ring-Opening Polymerization of <i>rac</i>-Lactides Catalyzed by Chiral and Achiral Aluminum Half-Salen Complexes

A new series of aluminum half-salen complexes have been synthesized from either chiral or achiral tridentate Schiff base ligands derived from amino alcohols or amino acids. All aluminum complexes have been shown to be active catalysts for the ring-opening polymerization (ROP) of rac-lactide in toluene at 70 °C, producing polylactides with controlled molecular weights and narrow molecular weight distributions. Both chiral and achiral aluminum complexes showed moderate selectivity to the ROP of rac-lactide to produce isotactic polylactide with a Pm value up to 0.76. In addition, epimerization of rac-lactide to meso-lactide was observed during the polymerization process for some of the complexes studied.本研究通过源自氨基醇或氨基酸的手性与非手性三齿希夫碱（Schiff base）配体，合成了一系列新型铝半salen（half-salen）配合物。所有该类铝配合物均具备催化活性，可在70℃的甲苯溶剂中催化外消旋丙交酯（rac-lactide）的开环聚合（ring-opening polymerization, ROP），所得聚丙交酯的分子量可控且分子量分布较窄。手性与非手性铝配合物对外消旋丙交酯开环聚合制备全同立构聚丙交酯均展现出中等立构选择性，其立构规整度参数Pm最高可达0.76。此外，在部分所研究配合物的聚合过程中，可观察到外消旋丙交酯向内消旋丙交酯（meso-lactide）的差向异构化现象。